1. Field of the Invention
This invention generally relates to power-saving measures for refrigeration systems having forced air evaporators which are used to cool enclosures under a variety of load conditions. In particular, the invention relates to reduction of heat input from the evaporator fans and motors to the enclosure during light load conditions.
2. Description of the Prior Art
Refrigeration systems for cooling refrigerators and freezers, employing evaporators having motor-driven fans, are well known. It is currently normal practice to leave the fan motors on the evaporator running at full speed whether the compressor is in operation or not. A recent innovation disclosed by U.S. Pat. Nos. 3,877,243 and 3,959,979 discloses means for operating the fan at full speed only when the compressor is operating and provides means for operating the evaporator fan at reduced speed when the compressor is off.
2. Brief Summary of the Invention
Refrigeration evaporators, which are used to cool enclosed spaces such as coolers or freezers, do not operate at full capacity all the time. They operate at full capacity only at those times that the compressors to which they are connected operate at full capacity. This occurs generally when the need for cooling is high or when the cooling thermostat directly or indirectly initiates compressor operation. At other times the compressor either operates at reduced capacity or is turned completely off.
Those skilled in the art of refrigeration know that high air velocity is necessary during periods when maximum cooling is required in order to provide adequate operation of the refrigerating evaporator. However, during off cycle, or non-refrigerating periods, only enough air need be circulated through the box in order to maintain a small temperature gradient throughout the interior of the box.
In fact, the inventors have determined that allowing high evaporator air flow to continue during periods of reduced cooling requirement, such as compressor off cycles, is unnecessary and wasteful of energy and harmful to the stored product in at least the following ways:
(A) The high air velocity increases the film coefficient between the air within the enclosure and the enclosure wall and thereby tends to increase heat flow from outside the enclosure through the wall and into the enclosure.
(B) When the compressor is not operating, or operating at reduced load, full power operation of all the evaporator fans requires the expenditure of substantial amounts of electrical energy, which is transmitted into the box as heat. In order to remove this heat from the enclosure, the compressor must operate for a sufficient period of time to do so, thereby consuming substantial additional power. Therefore, unnecessary operation of evaporator fans causes extra power consumption in two ways: (1) the power consumption required to run the evaporator fan, and (2) the power consumption required by the compressor to pump out the heat deposited in the box by the unnecessary running of the evaporator fans.
(c) High air velocity over the product tends to dehydrate the surface of the product, reducing the marketable product weight and causing a reduction in product quality.
The fraction of the fans which turn off when the capacity of the compressor is reduced or during compressor OFF conditions need not be pre-set. Where there are only two evaporator fans, then usually one will be turned off. If there are three evaporator fans, usually one or two will be turned off. In some cases, it may even be possible to turn off all the evaporator fans without any harmful effect when the compressor has been stopped by the operating control.
An advantage of shutting off some of the evaporator fans during compressor OFF cycle, leaving the remainder operating at full speed, is that full air throw is maintained. Air throw depends on air discharge velocity. When those fans remaining in operation continue to operature at their normal speed thus retain their rated discharge air velocity and their air throw remains unchanged.
The mechanism for turning off the evaporator fans may be directly related to the energization of the compressor motor or may be directly related to the condition of satisfaction or dissatisfaction of a cooling thermostat, or may be related to the unloaded condition of the compressor, or may be related to the condition of the device governing the degree of unloading of the compressor.
In an alternative construction, the fans are the directly controlled element and the compressor capacity control mechanism responds to the ON-OFF condition of the fans.